1. Field of the Invention
This invention relates to amplifier design, and, more particularly, to the design of Power Amplifiers.
2. Description of the Related Art
Electronic amplifiers are used for increasing the power and/or amplitude of various specified signals. Most amplifiers operate by sinking current from a power supply, and controlling the output signal to match the shape of the input signal, but having a higher amplitude. Amplifiers are typically specified according to their input and output characteristics. One of the main characteristics of an amplifier is its gain, which relates the magnitude of the output signal to the magnitude of the input signal. The gain may be specified as the ratio of the output voltage and the input voltage, or the ratio of the output power and the input power. The gain relationship is oftentimes expressed as the transfer function of the amplifier. In most cases, the transfer function of an amplifier is expected to be linear, that is the gain is expected to be constant for any combination of input and output signals. While linear amplifiers responds to different frequency components independently, and do not generate harmonic distortion, nonlinear amplifiers are oftentimes affected by distortion. Overall, if the transfer function or gain is not linear, the output signal may become distorted. There are many classifications addressing different amplifier design considerations, oftentimes defining particular relationships between the design parameters and the objectives of a given circuit. Various power amplifier circuit (output stage) classifications exist for analog designs (class A, B, AB and C for example), and for switching designs (class D and E for, example) based upon the conduction angle or angle of flow, Θ, of the input signal through the amplifying device—that is, the portion of the input signal cycle during which the amplifying device is conducting. The conduction angle is closely related to the amplifier power efficiency, and the image of the conduction angle may be derived from amplifying a sinusoidal signal (e.g. if the device is always on, Θ=360°.) Amplifier design typically requires a compromise between numerous factors, such as cost, power consumption, device imperfections, and a large number of performance specifications.
One widely used type of amplifier is the power amplifier, or ‘PA’. Power amplifiers are versatile devices that are used in various applications to meet design requirements for signal conditioning, special transfer functions, analog instrumentation, and analog computation, among others. One area where power amplifiers are typically used is in wireless applications. Such applications may feature a variety of RF (radio frequency) amplifier designs for use in the radio frequency range of the electromagnetic spectrum. RF amplifiers are oftentimes used to increase the range of a wireless communication system by increasing the output power of a transmitter.
Although it is generally desirable for the output of an amplifier to be a faithful reproduction of the input signal, as previously mentioned, this may rarely be the case due to inherent non-linearities of given amplifier designs and/or topologies. The effects of these nonlinearities may be kept to a minimum by keeping the input signal small. However, this solution is typically undesirable for power amplifiers, as it limits the output power level, and lowers the efficiency of the amplifier. One possible way to counteract distortion without having to contend with small input signals is through PA linearization techniques. One common PA linearization procedure relies on digitally predistorting the input signal to compensate for non-linearity effects. The predistorter typically manipulates both amplitude and phase of the input signal such that a predistorter stage and a PA stage cascaded together produce a linear output (except for saturation).
One example of a simple arrangement 100 of a predistorter and a power amplifier is shown in FIG. 1. Input signal ‘x’ is first fed through predistorter 102, generating a predistorted output signal ‘y’, which is then provided to power amplifier (PA) block 104 as the amplifier input, yielding undistorted output signal ‘z’. It should be noted that PA block 104 may include system components in addition to a PA. For example, the PA in PA block 104 may be preceded by linear stages such as baseband and PA driver circuitry. Most predistortion schemes rely on obtaining a transfer function, either from a table or using other methods. Power amplifiers are typically characterized in the lab, and the predistortion coefficients are programmed into a memory. For example, the PA may be simulated to obtain the coefficients, or physical testing may be performed on actual amplifier (oftentimes configured on integrated circuit chips) to obtain the coefficients, with the assumption that all PA chips from the same series will operate as desired using those coefficients. Testing is typically performed by injecting test signals/tones, e.g. a sinewave. These methods, however, may not be adequate to meet more stringent requirements in many systems.
Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.